The present invention relates to a speed control circuit that controls power to a furnace and air conditioner blower motor, which is typically an AC induction motor. The invention is especially directed to an arrangement for controlling blower motor speed to optimize the air flow into a heated or air conditioned space, with blower speed being at an appropriate speed for heating or air conditioning load. Preferably, the blower is operated in a manner to conserve electrical energy when the blower is operated at a reduced or lower speed, and which will fail-safe so as not to impair the safe pre-set operating speed for a heating mode.
In HVAC systems, such as home air conditioning systems, it is often desirable to change the fan speed or blower speed to control the amount of air flow through the system evaporator coil. Cold, dry air is considerably heavier than warm moist air, and so higher blower speed is needed for air conditioning than is needed for heating. In addition in the initial operation in an air conditioning mode, the blower has to operate at high speed to pump conditioned air, especially to higher floors. Then, when the comfort space or living space has cooled down, the fan speed can be reduced to avoid blowing cold air directly on human occupants. Reduced fan speed can also saves power and hence a reduced speed mode is often referred to as xe2x80x9ceconomyxe2x80x9d operation. Also, where sensible cooling is needed, rather than latent cooling (dehumidification) the blower is operated at higher speed to increase air flow. Correspondingly, if dehumidification is required rather than sensible cooling, the air flow rate should be reduced, requiring a slower blower speed. Other air conditioning load considerations can also create air flow requirements to govern blower speed, such as heat and humidity requirements for indoor plants, or preservation of expensive art works or musical instruments. The need for variable speed blower operation is discussed in my U.S. Pat. No. 6,118,239. That patent also discloses in detail a speed control drive circuit design for an inverter for powering the blower motor during a reduced-speed operation.
Blower motors for furnaces are designed with a fixed speed for a heating operation, and this speed is engineered to provide the proper amount of air flow through the furnace heat exchanger when there is flame present. This blower speed should not be changed, and reduced speed operation during heating could create a hazard due to possible overheating of the heat exchanger. On the other hand, the blower motor typically has the capability for higher speed operation during cooling or air conditioning. Conventionally, there is a full speed power input terminal on the blower motor, and a second terminal also, which is used for heating in which the motor operates at a pre-set lower speed. The HVAC furnace board has a thermostat relay that connects to the full speed power input terminal when there is a call for air conditioning, and to the second or heating terminal when there is a call for heat.
While it is highly desirable to provide a reduced speed blower capability for many possible air conditioning situations, it is important that this never interfere with the preestablished blower setting that is used for heating. It is also important that if there is a failure in the control mechanism, that the occupant be able to obtain heat, and if possible to obtain at least full-blower-speed air conditioning as well.
It is an object of this invention to provide a control circuit for an AC blower motor that avoids the drawbacks of the prior art as discussed above.
It is another object to provide a speed control or driver circuit that achieves efficient operation of a blower motor for air conditioning or dehumidification, but without adversely affecting blower operation during heating.
It is a further object to provide a control circuit that is will fail safe, and permit normal heating blower operation and full-speed air conditioning blower operation.
According to an aspect of the invention, a furnace and air conditioner control system involves a supplemental furnace board, i.e., supplemental circuit board or control logic, with circuitry for controlling application of electrical power to an AC blower motor. As discussed, the blower motor can have a cooling full speed power terminal and a heating (reduced speed) power terminal, as well as a common (or neutral) terminal. The blower motor is not necessarily a multi-tap motor, and single-speed motors are sometimes used on low-cost equipment, and the circuit arrangement of this invention can also be used to advantage with such motors. A main furnace board may typically be provided within or adjacent to the furnace/air conditioner assembly, and has a main AC input connected to the xe2x80x9cblackxe2x80x9d or xe2x80x9chotxe2x80x9d conductor of a pair of main AC conductors. A xe2x80x9cheatxe2x80x9d or xe2x80x9cfurnacexe2x80x9d terminal is connected to the heating terminal of the blower motor, and a xe2x80x9ccoolxe2x80x9d or A/C terminal is provided to connection to the cooling full-speed terminal of the motor. Generally, there is a main furnace relay connected to said main AC input, and a heating/air conditioning relay in series with said main relay. The latter has a normally-closed contact connected to the heat terminal and a normally-open contact connected to the cool terminal. There are also input terminals to which various thermostat conductors are attached, and circuitry responsive to thermostat signals e.g. a heat signal, a full speed cooling signal, and a partial speed cooling signal. In a preferred embodiment of the invention, the supplemental circuit board has a cool speed input connected to the above-mentioned cool terminal, a motor 1 terminal connected to the cooling full speed power terminal of the motor, a motor 2 terminal connected to the common terminal of said motor, and a neutral terminal connected to the neutral wire of the main AC conductors.
In order to provide xe2x80x9ceconomyxe2x80x9d operation, i.e., variable speed or reduced speed blower operation for air conditioning, a cool speed inverter circuit generates a reduced-speed AC power to operate the blower motor at a reduced speed when there is a call for partial speed cooling. The inverter circuit has first and second AC power inputs and a first power output terminal and a second power output terminal. On the supplemental board a first relay has normally-open contacts connected between the cool speed input and the motor 1 terminal, and normally-closed contacts connected between the first power output terminal of the inverter circuit and the motor 1 terminal. The second power terminal of the inverter is coupled to the motor 2 terminal. Other relay combinations are also possible.
Also, a second relay has normally-closed contacts connected between said motor 2 terminal and the neutral terminal, and normally-open contacts coupled between the neutral terminal and one AC power input of the inverter. The other AC power input of the inverter is coupled to the cool speed input. This second relay can be omitted in some circuit designs, as it serves mainly to prevent the inverter capacitors from overstress during times of normal motor speed operation.
A low-voltage logic circuit with inputs to receive thermostat signals may have an input to receive a blower motor in-use signal and has an output that provides a low-speed cooling signal, the latter being generated, e.g., when there is a call for reduced speed cooling or for dehumidification.
A first actuator circuit coupled to one or both of the cool speed input and the motor 1 terminal and has an actuator, e.g., relay coil, for actuating the first relay when AC electric power is being supplied to the blower motor. This first actuator circuit also generates the blower motor in-use signal when the blower motor is energized. This in-use signal generator can be omitted also, in some circuit designs. A portion of the first actuator circuit may be responsive to the low-speed cooling signal to disable the actuator or relay coil when the low-speed cooling signal is present. There is a second actuator circuit responsive to the low-speed cooling signal for actuating the second relay when said low-speed cooling signal is present.
Blower speed can be selected automatically based on heat or air conditioning load, or can be manually selected by the user, employing a selector or control mechanism which can be in an associated wall thermostat or supplemental input humidistat device. Blower speed can be anywhere in a range from low speed to high speed, or can be at two or more discrete values, e.g., low speed and full speed, or one or more speeds in between. In many cases, the inverter can gradually ramp up blower speed. The low-voltage logic circuitry can be microprocessor based. The inverter circuit can be a full bridge, half bridge, or other arrangement capable of delivering power efficiently to the blower motor for reduced speed operation. PWM drive systems can be employed, if desired.
The above and many other objects, features, and advantages of this invention will become apparent from the ensuing description of a preferred embodiment, which should be read in conjunction with the accompanying Drawing.